This invention relates to an improved method for installing elements for prestressing structures such as buildings, bridges and formed concrete structures of all types. In general, for the construction of a prestressed structure, tensioning strands are positioned in it during fabrication or construction and the strands subsequently are tensioned. The tensioning can be done before or after the concrete is poured forming the structure. There are numerous factors involved in determining whether a structure is to be built with previously stressed members, or whether the structure is to be stressed on site after fabrication.
When the structure is of a significant size, normally it is stressed at the building site. The size and weight of the parts of the structure prevent them from being prefabricated at a factory and transported to the building site. The structure must be poured and formed in final position and also tensioned in that position.
There are several ways of installing strands in the structure on situs. In one method, a conduit or duct is formed in the structure, normally by the placement of a hollow tube in the supporting frame before the concrete or similar material is poured. Subsequently, the requisite number of strands are pulled out of a supply reel, cut to the desired length, and placed side-by-side on the ground. One end of the group of strands is capped or otherwise rigidly fastened together, such as by welding. A guide wire is forced through the conduit and attached to the group of strands, and the strands are then pulled through the length of the conduit as one unit. Subsequently, the strands are stressed.
Such a method of prestressing, however, is time-consuming and expensive and also necessitates the use of heavy construction equipment and a significant amount of space. Each of the strands forming the bundle has to be taken from a supply reel and laid out straight so that the other cables can be placed next to it. The strands are heavy and sometimes construction equipment is necessary to position all of the cables in a bundle. Where the cables are of significant length, a similar amount of free space is necessary at the construction site for this procedure; such space at some construction projects is extremely limited, however. After the bundle or group of strands is formed, it necessary for a crane or other heavy construction equipment to transport the bundle, position it near the conduit, and assist in inserting it into and through the conduit. Thus, the use of numerous items of construction equipment and a team of construction workers is necessary for such construction projects.
In a second method of installing strands, the strands are prefabricated in a plant with all of the necessary hardware attached to them, such as bearing-plates, anchor-heads and the like. Groups of the strands are inserted in flexible conduits and coiled for delivery to the construction site. The strands are usually inserted in the flexible conduit in the same manner as described above, namely by the use of a guide wire and then by being pulled as a group through the conduit.
At the site, a coiled group of strands is placed on a turn table held by a crane and hoisted onto the structure. The coil is unwound and placed in position at the site. Concrete is then poured around the conduit, and allowed to set before the strands are stressed.
The "turn-table" method also requires the use of heavy construction equipment and a large team of workers. The turn-table is large and is cumbersome to move, position and operate at the construction site.
When the strands or tendons are stressed before concrete is poured forming the structure, it is normally carried out on a casting bed. The strands are pulled from a supply reel, laid out straight on the ground forming a bundle, and placed in position on the casting bed. Other requisite reinforcing steel inserts are then placed on the casting bed, forms are positioned around the work, and the concrete is poured.
Prestressing of the strands before the concrete is poured can only be utilized for small members and structures due to transport considerations. Also, significant anchorage is needed at the stressing site to accommodate the tremendous stressing forces.